According to the official Metroid Prime Web site, the power beam is referred to as “the basic energy unit”.  This would likely be understood as a joule.  The official Metroid Fusion Web site calls it an “energy beam”.  Energy comes in different forms, and I would suspect the power beam is at least using kinetic energy, since the object fired is in motion. Let’s go over the properties of the power beam. It’s able to knock back humanoids. This doesn’t necessarily mean the power beam has mass. Photons are massless, but still possess momentum. Second, the information about Eyons tells us the power beam produces a bright flash of light when it ignites.  To ignite means to cause something to catch fire. “Ignite” here probably means “explode”, since the power beam never sets anything on fire. Of course, it does leave charring on walls.
The next thing we can learn about the power beam is that it’s not considered a “natural energy”.  It could be artificial energy, or as a few have suspected, supernatural energy. If it’s supernatural energy, we must forfeit any understanding of the power beam by this point. If it’s artificial energy, then we can continue. I am inclined to think it’s artificial energy because the way it’s described seems to match a natural understanding. No doubt, in order for it to be such, it must be man-made. Plasma is found in nature and it can be produced unnaturally, but this kind of thinking would contradict the plasma, ice, and wave beam being natural, since these are produced unnaturally. My only guess to resolve this inconsistency is to say that the Chozo aren’t affected by heat, cold, and electricity.
The power beam uses several types of energy, such as kinetic (due to its motion), potential (because of its reference point), thermal (the motion of the particles), and gravitational energy (its association with the gravitational field), but this doesn’t necessarily mean this is what the power beam is made of. It’s doubtful that the power beam is electrical or sound energy because it shares neither of these properties. All we have left are chemical, elastic, electromagnetic, ionization, and nuclear energy. It could be possible that chemical energy is used to generate the power beam, just like a battery would produce light, but again, that would be what the power beam uses, not what it’s made of. I’m not so sure that the power beam would fall under elastic, ionization, or nuclear energy, however.
Considering Samus’ beam weapons fall under directed-energy weapons, I wouldn’t be surprised if electromagnetic energy was what the power beam was made of. More specifically, what would this beam actually be? Considering the power beam does not pass through solid objects like the wave beam, that would mean it’s being absorbed by the target. Gamma rays and radio waves can pass through solids, and so can visible light, but only if it’s powerful enough. We know the power beam isn’t powerful enough to pass through solid, unless it destroys the solid by smashing into it. For anyone who might be wondering, yes, I am proposing the idea that the power beam is a laser, or more specifically, a pulsed laser. What would this coincide with?
I think the power beam would best fit pulsed energy projectiles (PEP) because not only are the beams fired in bursts, they’re non-lethal. Of course the power beam can kill, but it’s the weakest of Samus’ beams. So the power beam could be a more powerful PEP. This is done by laser heating, which causes plasma to form on a target surface. The electrons in the plasma absorbs laser light through inverse Bremsstrahlung and heats the air. The plasma is heated strongly and a shock wave forms and moves away from the surface. Electron density is high enough to absorb all of the laser light in the shock front. Then high pressure air plasma exerts a very large mechanical force on the target. 
These properties are similar to the power beam, except the power beam is not referred to as plasma or a laser, and the projectile can be seen traveling, rather than waiting to hit a target before the results occur. An alternative could be a laser bullet.  Whether or not the power beam is anything like a PEP or PIKL, the one thing that stands out is that it is most likely made of electromagnetic energy. Why does the power beam have to be electromagnetic energy when it uses other types of energy? Considering directed-energy weapons emit high focused energy and these include electromagnetic radiation, particles, and sound, it seems to make the most sense for electromagnetic energy to be part of the power beam’s aspect.
Sure, bullets use kinetic, potential, thermal, and gravitational energy as well, but I’m not aware of these energies as objects that stand alone. The power beam is energy, not mass. I suspect this will make some question whether or not the power beam actually travels at the speed of light based on what we can see in game play, and I wouldn’t expect any less. However, I am not going to work strictly with the visual speed of projectiles in the Metroid series, considering there have been beams that are described as lasers or high-frequency beams, and yet travel slower than what the intended speed would be. From my observation, I think the closest thing the power beam would match is PEP. I honestly am not certain what else it could be at this point.